The present invention relates to devices for indicating tire inflation pressures and other parameters sensed on a rotatable wheel, and to devices which also sense brake wear.
Tire inflation pressure is a highly significant factor in promoting maximum tire life. An under-inflated tire will wear excessively on the edges whereas over-inflation will cause excessive wear at the center. Moreover, tire pressure affects the roadability of a vehicle since an under-inflated tire has a greatly increased "slip angle" and unequal tire pressures can, therefore, result in unpredictable and dangerous handling characteristics.
To measure the inflation pressure of each tire while a vehicle is stopped is a bothersome procedure often overlooked by otherwise prudent motorists, there being a tendency to rely upon visual inspection to determine whether tires appear "soft". Visual inspection is likely to detect only gross variations in tire pressure, particularly in the case of radial tires. Even extreme variations in pressure are difficult to detect visually when more than two tires are carried by a single axle, as in the case of large trucks and heavy construction equipment.
The need to make the driver of a moving vehicle aware of variations in tire inflation pressure takes on greatly increased importance with the advent of so-called "run flat" tires which are intended to allow the vehicle to run on a deflated tire, thereby eliminating the need for a spare tire. A deflated tire is, however, suitable only for relatively short distances at reduced speeds and the driver must be made aware of the need to affect a repair.
Safety problems associated with incorrect tire inflation pressures are particularly important in the case of aircraft, which are considered "vehicles" as that word is used herein. Demands made on aircraft tires are severe because of the ambient pressure variations that accompany changes in altitude and the high instantaneous loads sometimes produced during landings. The advantages to a pilot of being forewarned of incorrect inflation pressure are readily apparent. Despite the importance of inflation pressure, the pilot has not been provided with an indication of the parameter in state of the art aircraft. Failure to maintain correct pressure has been blamed for a number of recent aviation disasters.
In view of the importance of tire inflation pressure, a variety of pressure sensors have been proposed. Some earlier devices relied upon the direct connection of an air hose to the tire. These devices tend, however, to be heavy and a source of potential air leakage due to the difficulty of connecting a hose mounted on the vehicle chassis to the rotating wheel. Other proposed devices are electrically operated, relying upon sliding contact with sensor components mounted on the wheel. More recently proposed devices utilize electrical interaction between stationary, i.e., non-rotating, components and those mounted on the wheel without the necessity for sliding contact.
Devices of this last mentioned type, exemplified by a sensor described in U.S. Pat. No. 3,723,966, depend upon a force field generated by a stationary coil to produce a response in a resonant circuit mounted on the wheel. Some devices, such as that described in U.S. Pat. No. 3,092,806, employ two concentric coils, one rotatable with the wheel and the other stationary. The ferromagnetic axle and wheel-supporting structure at the center of one of these coils provide a high permeability path in which the magnetic flux tends to be concentrated, producing a field of low intensity in the area in which electro-magnetic interaction between rotating and non-rotating sensor components is intended to take place. Because this field is of low flux density, the clearance between the components must be very small and precise alignment is critical. It is difficult to maintain this alignment over a long period of use in view of the hostile environment and high shock loads received by the unsprung components of the vehicle. In addition, relatively high amplification of a weak signal is required to produce a usable output.
That the above devices have not satisfied the practical demands made upon a tire pressure sensor is apparent since they have not come into common usage. Such a device, to be practical, must be relatively simple, inexpensive and reliable without requiring frequent service. It must be sufficiently sensitive without being significantly effected by dirt, should not be fragile and should not require unduly precise alignment between the closely spaced components mounted on the wheel and the non-rotating vehicle structure. Components mounted on the wheel should be lightweight to avoid an out-of-balance condition and should not require a separate power source. If possible, the sensor should be capable of reading the inflation pressure while the vehicle is standing still, as well as when it is moving, since it will usually be desired to check the tire pressure before the vehicle is put into use, this being particularly true in the case of aircraft. It is also important that the pressure sensor not be a source of air loss.
The problems attendant reading of tire inflation pressure are similar to those that are involved in communicating other parameters that may be read on a rotatable wheel. Examples of such other parameters are axle deflection and tire side wall deflection.
An associated problem in the operation of vehicles is the provision of a signal that indicates the need for brake pad replacement. Brakes of this type used on automobiles, some aircraft and other vehicles utilize high friction brake pads attached by rivets to shoes which are connected to the vehicle frame so they cannot rotate. When the brake is applied, the shoe is moved to engage a smooth metal member in the form of a disk or drum that rotates with a wheel of the vehicle, the friction between the stationary pad and the rotating member providing the braking force. The friction surface of the pad is gradually worn away until the pad has been consumed and must be replaced.
If expensive repairs and brake malfunctions are to be avoided, it is imperative that the pads be replaced before they are worn to the point at which the rivets contact and score the rotatable member. The life expectancy of a pad may be predicted, within broad limits, based on the wear resistance of the material of which it is made, and the working thickness between its original friction surface and the heads of the rivets. Nevertheless, variations in individual patterns of use and driving habits make accurate prediction of replacement intervals impossible. Most vehicles, therefore, require frequent visual inspection of the brake pad, which is a time-consuming and inconvenient process, to guard against brake damage or loss of braking effectiveness due to pad wear. Failure to inspect brake pads with sufficient frequency is not unusual.
It would be highly desirable if a remote indicator responsive to the need to replace brake pads could be provided, eliminating the need for brake pad inspections, and various arrangements have been proposed to this end. Such arrangements have not, however, come into common use, primarily because of the expense and complexity of the components that must be added to the vehicle to perform this function alone.
An objective of the present invention is to provide a sensing system that overcomes the disadvantages of previously known systems, making additional information available to the operator of the vehicle, particularly making available informator relating to tire pressure. A further objective is to provide such an apparatus that is capable of indicating brake pad wear.